1. Technical Field
The invention relates to operating DC-linked AC converters connected in parallel for feeding an asynchronous machine and, more particularly, for driving equipment using the principle of field-oriented control.
2. Discussion of Related Art
It is commonly known to use DC-linked AC converters for feeding electrical machines at variable frequency with varying current and voltage. A DC-linked AC converter comprises a line converter, an intermediate current circuit with a DC smoothing choke and a machine converter. The converters are preferably arranged in a three-phase bridge circuit. The DC link current is adjusted and held constant with a suitable controller via the line converter. The DC link current is converted to AC with the desired frequency and distributed to the phases of the machine through the machine converter.
If the power of a DC-linked AC converter is not sufficient to supply a machine, then it is known to connect DC-linked AC converters of identical design in parallel, see for example EP 511 344 B1, EP 600 312 B1. With the solutions described therein, an almost sinusoidal motor current can be maintained in drives with a high rotation speed and high output power even at operating frequencies above 500 Hz; however, these solutions cannot be easily applied to drives which have to operate over a wide range of rotation speeds and with different torque requirements.